FIG. 1 schematically shows the basic design of a power supply device of the two-switch flyback type, which can be used for example to supply, from a constant supply voltage Vin, a load L comprising for example a LED light source, such as a so-called LED string.
According to a well-known topology the supply device, denoted on the whole by 10, is built around a transformer T having a primary winding T1 and a secondary winding T2.
Primary winding T1 is connected to voltage Vin on an input line denoted by 12 via two electronic switches 14a, 14b, adapted to be made conductive (“on”) and non conductive (“off”) in mutual alternation; in other words, when 14a is on, 14b is off and vice versa. In various embodiments, said switches are comprised of mosfets.
Each switch 14a, 14b has associated therewith a respective diode D3, D4, in an arrangement wherein:                switch 14a (called “high” switch) is interposed between the input line 12 and the cathode of diode D4, the anode whereof is coupled to circuit ground,        switch 14b (called “low” switch) is interposed between ground and the anode of diode D3, the cathode whereof is connected to input line 12, and        both ends of primary winding T1 of transformer T are respectively connected to the intermediate point between high switch 14a and diode D4 and to the intermediate point between diode D3 and low switch 14b.         
Secondary winding T2 of the transformer usually includes a rectifying/stabilizing network, which here is schematically depicted as a diode D1 and an output capacitor C, across which a voltage Vout is present which corresponds to the voltage across load L, towards which a current is supplied with intensity iout.
The basic arrangement in FIG. 1 meets operation requirements which are well known in the field, so as to make a detailed description unnecessary in the following.
The arrangement of the two-switch flyback converter in FIG. 1 shows, as compared with a single-switch flyback converter, the advantage that the maximum reflected voltage equals the main voltage, which supports a Zero Voltage Switching (ZVS) without applying high voltage to the mosfets.
This is a remarkable advantage, because it makes it possible to use mosfets with lower drain-source voltage values, which are both more efficient and less costly.
The arrangement in FIG. 1 has however a drawback in that the high switch 14a has a gate floating with respect to ground, which may cause driving difficulties and consequently a possible efficiency drop and a lower switching speed.
The inventors have observed that this problem may be solved, at least theoretically, by resorting to auxiliary supply sources connected to the high side of the circuit.
This solution would however involve the provision of a further supply source within the same circuit, with an undesirable addition of components and higher costs, due for example to resorting to a driver that can withstand voltages as high as 500-600 V: such components, albeit available, tend to be rather costly.
These factors are not compatible with the presently considered solutions, which, particularly for consumer market applications, must have a cost and a power absorption as low as possible.
Moreover, the inventors have observed that another possibility could involve the use of a pulse transformer. This would result again in a rather expensive solution, because both the transformer and a driver, adapted to operate with the high currents required by a pulse transformer, would naturally be quite costly, and would limit the use of such a solution to high power applications in professional sectors. Moreover, a solution involving the provision of a pulse transformer would impose restrictions on the duty cycle as well (which cannot be higher than 50%).